ROHM driver IC solves LED lighting problems

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Foreword

Nowadays, from home to industry, people's environmental awareness has been widely improved. Therefore, LED lighting lamps with energy-saving effects are rapidly becoming popular as replacements for original lighting fixtures (incandescent bulbs, fluorescent lamps, etc.).

As LED lighting, it needs to have a high-power product power source capable of driving small power products such as small lamps such as light bulbs to large-scale lighting such as street lamps. Moreover, this kind of power supply should be compatible with "controllable silicon dimming" used in original lighting fixtures, "PWM dimming" for LED lighting, and "linear dimming" which can be dimmed by bulk resistance. compatible. In order to illuminate and dim the LED stably, an LED driver IC is required, and the driver IC is required to be compatible with various power sources such as power and dimming methods.

Features of ROHM LED lighting driver IC

In order to meet the above market demand, ROHM is developing driver ICs for LED lighting. In order to keep the LEDs at a certain brightness, an LED driver IC for constant current control is required. Normally, constant current control is started from the AC power supply, so the step-down switching regulator method is used (Fig. 1).



[Fig. 1] Common lighting power module

In the conventional method, the input voltage variation and the output voltage fluctuate greatly, and the power module must be redesigned according to the application conditions. If the input voltage fluctuates greatly, the rippled input voltage in the AC voltage frequency (100 Hz to 120 Hz after full-wave rectification) causes the brightness of the LED to flicker. In order to stabilize the input voltage, measures must be taken when designing the power module. If the output voltage fluctuates greatly, it is necessary to change the constant of the external component for each output voltage of the light source (the number of LEDs), and design a variety of power supply modules, thereby increasing the design workload (Fig. 2). Moreover, due to the varying Vf of the LEDs, the brightness of the LED lighting fixtures may be inconsistent.

[Fig. 2] Input voltage characteristics



[Fig. 2] Output voltage characteristics

The switching regulator uses the ratio of the on-time to the off-time of the switch to control the LED current value. The conventional method of general control is to use the peak current of the coil to determine the on-time of the switch. This method uses the change in input and output voltages to change the gradient of the coil current. As a result, the LED current value also changes. (Figure 3-1)

[Fig. 3-1] LED current waveform (peak detection control)

[Fig. 3-2] LED current waveform (average control)

To solve this problem, the peak current should not be used to control the LED current, but must be controlled by the average value of the LED current (average current control). However, the average current control is difficult to implement with the analog power control method generally used, and the number of external components tends to increase. The BD555AKFV developed this time uses digital power control technology, which does not increase the number of external components, and only needs to detect the current of the coil to solve the above problems. (Figure 4)

[Fig. 4] Block diagram of driver IC for LED lighting with thyristor dimming BD555AKFV

Due to the digital power control, the average value of the LED current can be controlled regardless of the input and output voltages (Figure 2). It digitizes the coil current (A/D conversion) and performs peak current calculation based on the sampled data to keep the average value of the LED current constant. Then, the operation result is D/A converted and fed back to the comparison circuit for detecting the peak current (Fig. 3-2). Rom uses this method to fully implement digital control, which is the first in the industry.

Although the digital power control method adopted by the switching power supply has attracted attention, it is difficult to apply to LED small-sized lighting because of the problems of "large circuit scale (high cost)" and "high power consumption" due to the use of the DSP. BD555AKFV has developed special logic for LED lighting functions, which solves the weakness of digital power supply - the problem of increased circuit and power consumption.

The digital power control feature is effective in utilizing the dimming function in addition to the average current control function. "BD555AKFV" is compatible with "dimmable silicon dimming", "PWM dimming" and "linear dimming" three dimming methods, and can be adapted to lighting fixtures with various dimming functions.

Driver IC for LED lighting with thyristor dimming function

There is a need in the industry to combine LED lighting with dimmers used in existing lighting fixtures. The original dimmer is a thyristor dimmer that uses a switching element such as a thyristor to intercept a part of the AC input voltage and adjust the brightness according to its phase angle. The thyristor dimmer must keep the holding current flowing while the thyristor is in the conducting state; once the current is kept down, the thyristor is turned off. Since the LED lighting lamp has extremely low power consumption compared with the original lighting lamp, the holding current of the thyristor is lowered, and the dimmer is often turned off without periodicity to cause flicker (flicker). (Figure 5)


[Fig. 5] Waveform when the thyristor malfunctions

In order to prevent such flickering, a bleeder circuit capable of ensuring a constant current is required, and each manufacturer constructs a bleeder circuit in various ways. The BD555AKFV incorporates ROHM's original bleeder circuit and takes measures to avoid deadweight losses. However, even if a bleeder circuit that ensures a constant holding current is loaded, there is a case where the malfunction of the thyristor dimmer cannot be prevented. The reason is because the rush current that charges the input capacitor when the thyristor is turned on causes a large transient, keeping the current drop instantaneously and causing the thyristor to turn off (Figure 6).

[Fig. 6] The forward current when the thyristor is turned on

Taking measures to prevent transients will result in a decrease in efficiency. This issue cannot be easily solved. Roma's response to sudden transients is to use digital controls to avoid flicker. The conventional method is to input a thyristor phase angle detection voltage that has been smoothed through a capacitor to a dimming decoder. Although the capacitor has the effect of filtering out the malfunction caused by the transient, it cannot completely eliminate the malfunction caused by the low frequency region of 100Hz/120Hz, or there is flicker. The BD555AKFV has a digital filter inside the IC to solve the above problems. The BD555AKFV samples the phase angle and stores the digital samples in memory. Before adjusting the dimming brightness, the sample is completely shielded from the error generated by the sudden malfunction of the thyristor dimmer by a digital filter designed specifically for thyristor dimming to prevent flicker. (Figure 7)

[Fig. 7] LED current waveform of BD555AKFV when the thyristor dimmer malfunctions

from now on

ROHM has developed LED driver ICs that are most suitable for a wide variety of LED lighting fixtures, contributing to the popularity of LED lighting. Based on the digital power technology and dimming technology introduced this time, we plan to use various technologies to form a product line with a variety of built-in power improvement circuits that can be selected by users for various applications.